The present invention concerns a procedure and an apparatus for determining the geometrical shape of a surface, in particular on work pieces to be machined, welded, measured, or inspected.
In mechanical industry, apparatus of various kinds are in use for measuring the surface geometry of work pieces. Such apparatus usually serve for inspection of finished pieces, or for process control purposes, as for example automatic machining by means of so-called adaptive control. In welding technology such surface indications are useful for automatic guidance of the welding head along the joint (joint tracking), for controlling the operating parameters (for example current, voltage, speed etc.) of the welding equipment during so-called process control, and for inspecting the finished weld with regard to appearance and quality.
For some industrial applications photoelectric equipment is available, which usually comprises a TV-camera, and which is employed in such manner that the location of lines of contrast (transitions between light and shadow) in the TV picture may be registered by means of indicator instruments, ink recorders, or computers. This implies for example the possibility of measuring certain dimensions and positions of dark regions in a TV picture, which otherwise is bright. This principle based upon photoelectric surveillance of illuminated surfaces is employed in mechanical industry in various ways, for example:
A. By turning in a lathe, a TV camera is focussed on the work piece which is seen against a bright background, giving an outline of the work piece. Feeding of the cutting tool in the lathe is done on the basis of electric output signals from the TV camera, which represents the contrast outline.
B. In Soviet technical literature, equipment for automatic joint tracking during welding has been described, which employs a light source placed behind the open joint and a TV camera in front focussed on the bright slit. The contrast lines in the TV picture are in that case the edges of the bright slit. The welding head is tracked automatically along the slit displayed, which means along the joint to be welded.
c. U.S. Pat. No. 3,532,807 also shows guidance of a welding head along a joint to be welded. In this case, one edge of the joint is processed in advance, to reflect incident light into a TV camera, which thus will register a bright line along the joint, that is parallel to the motion of the center line of the welding head.
The major disadvantages of the known devices of this kind are that they do not in many important practical cases supply sufficient information with regard to the shape of the surface. It is limited to the location of particular lines which form natural borders between physically separate areas, for example the border between bright and dull surfaces (point c. above) or the outlines of a work piece against a background (a. and b. above). These known devices do not supply any information, however, with regard to the shape of the surface, on such parts of the piece which do not contain a definite edge as a natural line of contrast. For example, such devices do not supply any information with regard to the cross section of the joint to be welded, as seen in a direction normal to the direction of the weld.
In many cases the devices just described are also unpractical. They require either surface preparation (c.) or access to the region behind the work piece (a. and b.)